Load Balancing Terms

Connection

A connection is established when one host contacts another host with the desire to send/receive information via the transport layer. First, the hosts establish connection, identifying one another and where they each sit on the network or Internet, then data transfer occurs. The hosts send IP packets to one another in a mutually organized manner. Finally, the connection is released, completing the transaction. These three steps ensure that the data arrives in order, without corruption.

Connection-Less Protocol

Data transmission occurs between two hosts that have not previously set up a connection. This method can save bandwidth, but is more prone to errors than connection oriented protocols. See Packet Switching. Distributed Network: A network with nodes in multiple locations, such as an ISP with a node in Tokyo as well as one in New York City.

DNS

A Domain Name Server (DNS) resolves domain names to their equivalent IP addresses so that IP traffic can be transported to the correct destination. Each Domain Name (i.e., informationweek.com) is associated, at a minimum, with a Primary and a Secondary DNS. Domain Name Servers are located throughout the Internet. Many ISP's maintain their own DNS servers to reduce their use of bandwidth. Occasionally, these local DNS become out of date and lead to browser errors.

Domain Name

See DNS.

Failover

If a network is equipped with redundant resources, such as mirrored servers or tandem load balancers, the secondary device can assume the duties of the primary should the primary fail. This can be done manually or automatically depending on the setup.

Fault Tolerance

A network that is fully redundant and has maximum up-time is considered fault tolerant. In other words, should a component of the network fail, the network will continue processing requests and the failure will be transparent to clients.

FTP

File Transfer Protocol (FTP) is based on TCP/IP and is used to transfer files from one site to another via the Internet or through a LAN or WAN. Host: A fancy word for a computer on a network. This includes a user or server across the Internet or in the same office. The hostname of a server on the Internet also includes the domain name to make a complete Internet address. For instance, if a machine is known locally as server1 and is part of the domain at yahoo.com, the full hostname is server1.yahoo.com.

HTTP

Hypertext Transfer Protocol is the medium by which web pages (HTML documents) are distributed via the Internet. Some load balancers do an "HTTP poll" to determine server health on a network. In this case, the load balancer requests a specific HTTP page; if the page is okay, the server is deemed healthy. If, for some reason, the page does not respond properly, the server is deemed inactive and no more traffic is sent to it.

IP Address

A 32 bit designation for a host, that consists of a network address, host number, and subnet mask. These are written in dot notation (i.e., 206.144.2.3) that actually corresponds to numbers of bits. IP addresses can be in one of three classes: A, B, and C. They can be "legitimate" or "non-legitimate". Legitimate addresses are public, such as those used for web sites, whereas non-legitimate addresses are given to devices behind the scenes, such as web servers reached through a proxy device such as a firewall.

IP Packet

A segment of information that maintains its integrity as it travels through the Internet or across a network. Each packet contains header information that includes the type of traffic it is (FTP or UDP, for example) and where it originated from. SNMP can be used to track packets to gather a variety of statistics on network usage and load. The header information is also used by the responding server/load-balancing device to ensure that the answering packet goes to the correct location.

Load Balancing

The process by which load (number of requests, number of users, etc.) is spread throughout a network so that no individual device becomes overwhelmed by too much traffic, causing it to fail. Load balancing also involves redirection in the case of server or device failure to allow for Failover and promote Fault tolerance.

Packet Switching

During transport from one host to another, packets may be routed out of order and across a variety of paths to get to the desired end point. UDP uses this method of transport. It is up to the computer at the destination to reassemble the packets into the original order.

NAT

Network Address Translation (NAT) is used on devices such as firewalls and certain load balancers to allow clients to request information of a public IP address (assigned to a web site, for example) for which the information is actually located on servers with non-legitimate addresses (such as a group of web servers in a farm located behind the firewall or load balancer.

Persistent Connection

Also known as a "sticky connection". For instance, when a user accesses www.zerowait.com, the browser downloads the index.html file, plus all the images and buttons referenced in the HTML file. The retrieval of all of these individual page elements in HTTP may be performance enhanced by having the browser/server communicate with a single long session for several page elements, instead of a series of small sessions as each element downloads

Session

A lasting connection, usually involving the exchange of many packets between a host and a server.

Ping

The Packet Internet Groper is used to test the availability of a host on a network or on the Internet. A UNIX command, ping also works in Windows through TCP/IP. In DOS/WINDOWS, at the DOS prompt, enter PING and the IP address or the domain name of the server you want to test. If you get replies the server is up. If not, it may be down or there may be a problem between you and the server (such as a downed phone line). To test this, perform the command TRACERT with the same IP address or domain name

Port

Because most network interfaces have only one or two physical ports (the means by which data comes into the computer from outside), you need to designate port numbers for different kinds of IP traffic. For example, port 80 is commonly used for HTTP traffic and port 21 is used for FTP.

Redirection

The process by which traffic that is destined for one location is sent to another. This can be from server to server or, across a distributed network, from one WAN or LAN to another. Redirection is frequently used as a method of Failover.

Server

A process that runs on a host that relays information to a client upon the client sending it a request. Servers come in many forms: application servers, web servers, database servers, and so forth. All IP-based servers can be load balanced. See Web Server.

Server farm / cluster

A group of servers (usually four or more) that mirror one another or otherwise act as backup for one another or another farm. Spoofing: When a device, such as a load balancer, answers a request in the name of another device, such as a web server in a farm located behind it.

SuperFarm

A server farm containing multiple server farms. SNMP Management: Simple Network Management Protocol (SNMP) is a method by which you can generate reports and monitor your network and the equipment on it. Generally, SNMP works on the TCP/IP level.

TCP/IP

Transmission Control Protocol over Internet Protocol. These are two transmission protocols that work together to help the servers, clients and devices on the network talk to one another. FTP, HTTP, UDP, SNMP, and telnet run on top of TCP/IP.

UDP

User Datagram Protocol transports data as a connectionless protocol, using packet switching.

Web Server

Although it might seem that a web server is a machine, it is really a process running on a machine that serves HTTP content to web browsers on client machines. This is important because you can actually put several web servers on a single machine as long as you designate specific ports through which traffic will travel. Along the same lines, there are also FTP servers, mail servers, and so on, each of which handles a specific type of traffic.